[tools] Fix plugin_ffi template lint violation (#115356)

Closes: https://github.com/flutter/flutter/issues/115347

packages/flutter_tools/templates/app/lib/main.dart.tmpl

@@ -208,7 +208,7 @@ class MyApp extends StatefulWidget {
   const MyApp({super.key});
 
   @override
-  _MyAppState createState() => _MyAppState();
+  State<MyApp> createState() => _MyAppState();
 }
 
 class _MyAppState extends State<MyApp> {